In recent years, public attention has been focused on a photoelectric converter that converts light to electricity by utilizing a photoelectric effect inside a semiconductor, and development of the device has been vigorously carried out. Among these developments, a silicon-based thin-film photoelectric converter has been promising as a device that can achieve low costs, because it can be formed on a glass substrate or a stainless substrate with a large area, at low temperatures.
This silicon-based thin-film photoelectric converter generally has a structure in which a transparent electrode layer, one or more photoelectric conversion units and aback electrode layer are successively stacked on a transparent insulating substrate. In general, the photoelectric conversion unit has a p-type layer, an i-type layer and an n-type layer that are stacked in this order or in an order reverse to this. When the i-type photoelectric conversion layer, which occupies its main portion, is made of an amorphous material, the unit is referred to as an amorphous photoelectric conversion unit, and when the i-type layer is made of a crystalline material, the unit is referred to as a crystalline photoelectric conversion unit.
The photoelectric conversion layer is a layer that absorbs light and then generates an electron and hole pair. In general, in such a silicon-based thin-film photoelectric converter of a pin junction, the i-type layer serves as a photoelectric conversion layer. The i-type layer serving as the photoelectric conversion layer occupies a main portion of the film thickness of the photoelectric conversion unit.
Ideally, the i-type layer is an intrinsic semiconductor layer containing no conductivity type determining impurities. However, even when a layer contains a trace amount of impurities, it functions as an i-type layer of the pin junction when its Fermi level is located substantially in the center of a forbidden band; therefore, this is referred to as a substantially i-type layer. In general, the substantially i-type layer is formed without adding a conductivity type determining impurity to a raw material gas. In this case, the conductivity type determining impurity may be contained therein within a permissible range that allows the layer to function as the i-type layer. Further, the substantially i-type layer may be formed by intentionally adding a trace amount of conductivity type determining impurity so as to remove influences given to the Fermi level by impurities derived from the atmospheric air and underlying layer.
As a method for improving the conversion efficiency of the photoelectric converter, a photoelectric converter having a structure referred to as a stacked-type, in which two or more photoelectric conversion units are stacked, has been known. In this method, a front photoelectric conversion unit containing a photoelectric conversion layer having a large optical forbidden band width is placed on a light incident side of a photoelectric converter, and behind this, a rear photoelectric conversion unit containing a photoelectric conversion layer (for example, made of an Si—Ge alloy or the like) having a small optical forbidden band width is successively placed so that a photoelectric converting process covering a wide wavelength range of incident light can be obtained; thus, by effectively utilizing incident light, the conversion efficiency for a converter as a whole can be improved.
In the case of a double-junction type thin-film photoelectric converter in which, for example, an amorphous silicon photoelectric conversion unit and a crystalline silicon photoelectric conversion unit are stacked, wavelengths of light that can be photoelectrically converted by the i-type amorphous silicon (a-Si) are limited to about 700 nm on a long wavelength side; however, the i-type crystalline silicon can photoelectrically convert light with wavelengths longer than those, that is, up to about 1100 nm. Here, an amorphous silicon photoelectric conversion layer made from amorphous silicon has a relatively greater light absorption coefficient, and thus a film thickness of about 0.3 μm is enough to absorb light that is sufficient for photoelectric conversion. On the other hand, the crystalline silicon photoelectric conversion layer made from crystalline silicon has a relatively smaller light absorption coefficient in comparison with that of amorphous silicon, and thus its thickness is preferably set to about 2 to 3 μm or more so as to sufficiently absorb light having long wavelengths as well. That is, the crystalline silicon photoelectric conversion layer normally needs to have a thickness that is about 10 times larger than that of the amorphous silicon photoelectric conversion layer. Additionally, in the case of this double-junction type thin-film photoelectric converter, the photoelectric conversion unit located on the light incident side is referred to as a top cell, and the photoelectric conversion unit located on the rear side is referred to as a bottom cell.
In addition, a triple-junction type thin-film photoelectric converter provided with three photoelectric conversion units may also be used. In the present specification, the photoelectric conversion units of the triple-junction type thin-film photoelectric converter are referred to as a top cell, a middle cell and a bottom cell, in succession from the light incident side. By using the triple-junction stacked-type thin-film photoelectric converter, the open circuit voltage (Voc) can be made higher, with the short circuit current density (Jsc) being set lower, so that in comparison with the double-junction type, the film thickness of an amorphous silicon photoelectric conversion layer of the top cell can be made thinner. For this reason, it is possible to suppress photodegradation of the top cell. In addition, by making the band gap of the photoelectric conversion layer of the middle cell narrower than that of the top cell, and also wider than that of the bottom cell, incident light can be utilized more effectively.
As an example of the triple-junction stacked-type thin-film photoelectric converter, a thin-film photoelectric converter in which amorphous silicon germanium (a-SiGe) is used for the photoelectric conversion layer of the middle cell, with an a-Si photoelectric conversion unit/an a-SiGe photoelectric conversion unit/an a-SiGe photoelectric conversion unit being stacked in this order, or a thin-film photoelectric converter, with an a-Si photoelectric conversion unit/an a-SiGe photoelectric conversion unit/a crystalline silicon photoelectric conversion unit being stacked in this order, may be proposed. By appropriately adjusting the Ge concentration of the a-SiGe film, the band gap of the i-type a-SiGe of the photoelectric conversion layer of the middle cell can be controlled to a value between those of the top cell and the bottom cell. In the case where the a-SiGe photoelectric conversion layers are used for both of the middle cell and the bottom cell, the Ge concentration of the bottom cell is preferably made higher than that of the middle cell.
However, it has been confirmed that in comparison with a-Si, the a-SiGe layer, which is an alloy layer, has a higher defect density with its semiconductor characteristics being deteriorated, and also has an increase in defect density due to irradiation with light. For this reason, the triple-junction stacked-type thin-film photoelectric converter in which a-SiGe is used for the photoelectric conversion layer of the middle cell or the bottom cell is insufficient in improving the efficiency, in comparison with the double-junction thin-film photoelectric converter. Moreover, since the photodegradation of a-SiGe is serious, a problem arises that, although the triple-junction stacked-type thin-film photoelectric converter is used, it is not possible to sufficiently suppress the photodegradation.
In the case where the a-SiGe photoelectric conversion unit is used as the bottom cell, the wavelength of light that can be photoelectrically converted is limited to about 900 nm in a long wavelength side, and in the case where the amorphous silicon photoelectric conversion unit is used as the bottom cell, the wavelength of light that can be photoelectrically converted is limited to about 1100 nm in the long wavelength side. Therefore, the wavelength to be utilized on the long wavelength side is the same as the wavelength of a double-junction thin-film photoelectric converter, which fails to provide improvements, resulting in a problem that the improvement of the conversion efficiency of the triple-junction thin-film photoelectric converter is insufficient.